I know System.gc() is not guaranteed to cause GC, but theoretically, in the following code, will the object obj be eligible for garbage collection?
public class Demo {
public static void main(String[] args) throws Exception {
SomeClass obj = new SomeClass();
ArrayList list = new ArrayList();
list.add(obj);
obj = null;
System.gc();
}
}
class SomeClass {
protected void finalize() {
System.out.println("Called");
}
}
At the point where you call System.gc() the SomeClass instance you created is not eligible for garbage collection because it is still referred to by the list object, i.e. it is still reachable.
However, as soon as this method returns list goes out of scope, so obj will then become eligible for garbage collection (as will list).
Simply setting the reference obj to null does not, by itself, make the object referred to eligible for garbage collection. An object is only eligible if there are no references to it from the graph of visible objects.
will the object obj be eligible for garbage collection?
Only those objects are garbage collected who don't have even one reference to reach them. (except the cyclic connectivity)
In you code, there are two reference that are pointing to new SomeClass();
obj
zeroth index of list
You put obj = null, i.e. it's not pointing to that object anymore. But, still there exists another reference in list which can be used to access that object.
Hence the object will be eligible for GC only when main returns. i.e. you can't see the output of finalize method even if it got called. (not sure if JVM still calls it)
No, because the object actually exists in the list.
You as Java programmer can not force Garbage collection in Java; it will only trigger if JVM thinks it needs a garbage collection based on Java heap size
When a Java program started Java Virtual Machine gets some memory from Operating System. Java Virtual Machine or JVM uses this memory for all its need and part of this memory is call java heap memory.
Heap in Java generally located at bottom of address space and move upwards. whenever we create object using new operator or by any another means object is allocated memory from Heap and When object dies or garbage collected ,memory goes back to Heap space in Java
EDIT :
will the object obj be eligible for garbage collection?
No, because the object is still in the ArrayList.
Agreed, it won't be garbage collected as long as its there in the list.
Related
My concern is about an instance of an object that was once strongly referenced, but after an explicit null assignment to its strong reference and after an explicit System.gc() call, the instance is still reachable via the weak reference. If I understand correctly, when a referred object has only weak references left, the referent is guaranteed to be cleared in the next GC session. What am I missing?
Reference code:
public class References {
public static void main(String[] args) {
Example strongReferenceWrappedInWeak = new Example(42);
strongReferenceWrappedInWeak.printA();
WeakReference<Example> exampleWeakReference = new WeakReference<>(strongReferenceWrappedInWeak);
System.gc();
Example retrievedExample = exampleWeakReference.get();
retrievedExample.printA(); //this works, because a strong reference is present to the instance, hence it's not cleared
strongReferenceWrappedInWeak = null; //eligible for garbage collection
System.gc();
Example retrievedExampleTwo = exampleWeakReference.get(); //should be null
retrievedExampleTwo.printA(); //should throw NPE
}
}
class Example {
private int a;
Example(int a) {
this.a = a;
}
void printA() {
System.out.println(this.a);
}
}
strongReferenceWrappedInWeak = null does not make the Example object instance eligible for garbage collection, because retrievedExample still maintains a strong reference to it.
To fix, add retrievedExample = null;
Example strongReferenceWrappedInWeak = new Example(42);
strongReferenceWrappedInWeak.printA();
WeakReference<Example> exampleWeakReference = new WeakReference<>(strongReferenceWrappedInWeak);
System.gc();
Example retrievedExample = exampleWeakReference.get();
retrievedExample.printA(); //this works, because a strong reference is present to the instance, hence it's not cleared
retrievedExample = null;
strongReferenceWrappedInWeak = null; //now eligible for garbage collection
System.gc();
Example retrievedExampleTwo = exampleWeakReference.get(); //will be null
retrievedExampleTwo.printA(); //will throw NPE
Alternatively, don't create a strong reference with a local variable, just call the method directly off the weak reference. That way you don't accidentally leave a strong reference behind, as you did. *(During the printA() call, the this reference is a strong reference, so object cannot be GC'd during the call)*
Example strongReferenceWrappedInWeak = new Example(42);
strongReferenceWrappedInWeak.printA();
WeakReference<Example> exampleWeakReference = new WeakReference<>(strongReferenceWrappedInWeak);
System.gc(); //does not collect object, since strong reference still exists
exampleWeakReference.get().printA(); //works
strongReferenceWrappedInWeak = null; //eligible for garbage collection
System.gc(); //collects object, since it is now weakly referenced only
exampleWeakReference.get().printA(); //throws NPE
Output (from both)
42
42
Exception in thread "main" java.lang.NullPointerException
at Test.main(Test.java:**)
Tested on Java 13
Garbage Collection works in mysterious ways.
There have been several implementations of garbage collectors in the Java ecosystem, with very different behaviors.
When garbage collection runs varies by the implementation of the garbage collector, and also may depend on the current condition of the JVM. One collector might run almost continuously, while another might wait until low on memory. (I’m greatly oversimplifying here, to make the point clear.)
Whether all garbage is collected, or just some of it, may also vary by collector implementation and by the state of the JVM.
The call to System.gc is merely a suggestion, not a command. A garbage collector is free to ignore it.
In Java, you should not be making any great effort at managing memory. The modern JVM implementations are far better at that than any single programmer is likely to be. Just be sure to release all references to your objects when done using them. Or use WeakReference/SoftReference. Then trust the JVM and garbage collector to do its job.
In extreme cases (very large memory, or extreme volumes of object churn) you might want to study the behaviors of various garbage collector implementations. And maybe consider alternatives such as Zing from Azul Systems or GraalVM from Oracle. But for most projects, the usual OpenJDK-based JVMs work quite well.
As far as I know objects are available to be garbage collected when assigning a null value to the variable :
Object a = new Object;
a = null; //it is now available for garbage collection
or when the object is out of scope due to the method's execution is done:
public void gc(){
Object a = new Object;
} //once gc method is done the object where a is referring to will be available for garbage collection
given with the out of scope isn't also the same when the application just ended?
class Ink{}
public class Main {
Ink k = new Ink();
public void getSomething(){
//method codes here
}
public static void main(String[] args) {
Main n = new Main();
}
}
where I expect 2 objects (Ink object and Main object) should be garbage collected when the application ends.
When the Java application terminates, the JVM typically also terminates in the scope of the OS, so GC at that point is moot. All resources have returned to the OS after as orderly a shutdown of the JVM as the app defined.
You are confusing the event of an object becoming eligible for garbage collection with the actual process of collecting garbage or, more precisely, reclaiming memory.
The garbage collector doesn’t run just because a reference became null or an object went out of scope, that would be a waste of resources. It usually runs because either, memory is low or CPU resources are unused.
Also, the term “garbage collection” is misleading. The actual task for the JVM is to mark all objects being still alive (also known as reachable objects). Everything else is considered reclaimable, aka garbage. Since at the termination of the JVM, the entire memory is reclaimed per se, there is no need to search for reachable references.
That said, it’s helpful to understand, that most thinking about the memory management is useless. E.g. in your code:
public void gc(){
Object a = new Object;
// even here the object might get garbage collected as it is unused in subsequent code
}
the optimizer might remove the entire creation of the object, as it has no observable effect. Then, there will no garbage collection, as the object hasn’t been created in the first place.
See also here.
JVM monitors the GC roots - if an object is not available from a GC root, then it is a candidate for garbage collections. GC root can be
local variables
active java threads
static variables
jni references
duplicate - How to destroy java objects?
My question is very simple. I am new to app development with Java and am not sure whether I need to null objects after i am finished with them. for example i am using libGDX to create a game and I have several objects (Actors) being created. when I am finished with them do I simply call
obj.remove();
or should i -
obj.remove();
obj = null;
do I save memory by nulling objects or is there no advantage to be had?
No you do not need to null or manually delete objects. The java garbage collector will do this for you for any objects that have no pointers referencing them (when an object goes out of scope for example).
Generally, in java, marking the Object references as null is done to make it explicitly eligible for GC. If an object is unreachable, then it becomes eligible for GC, so, yes, you can mark it as null and let the GC do its work.
The Object will become unreachable only when there is no reference pointing to it.
example :
class MyTest {
#Override
protected void finalize() throws Throwable {
System.out.println("object is unreachable..");
}
}
// In some other class
public static void main(String[] args) {
MyTest o1 = new MyTest();
MyTest o2 = new MyTest();
System.gc();
o1 = null;
System.gc();
System.out.println("hello");
}
O/P:
hello
object is unreachable..
Here, you might have several thousand lines of code after "hello". You might want to make the GC's job easier by marking the object's references as null.
Manually nulling Objects in Java is bad, because it is slowing down most garbage collection (GC) algorithims. The GC detects by itself wether an Object is reachable or not and then it gets removed. After nulling an object the space in memory is still used and only after the GC recycles the space it can be used again. So nulling objects does not free up your space immediately. Also starting the GC manually is a bad idea. It is started by the VM if it is needed.
I have been, without a question, using the final keyword for years to denote fields that should not change for the lifetime of an instance/class. Suddenly this occured to me...
So given this example:
public class TestFinalGC{
private TestFinalGC(){}
private final Object obj = new Object();
public static void main(String []args){
TestFinalGC instance = new TestFinalGC();
// instance Ref -> actual instance ->? obj ref-> actual obj
System.out.println(instance.obj);
instance = null;
//say GC makes sweep here... what happens?
//lets assume theres more code, obj would obviously be eligible for GC on app exit.
}
}
How does the obj member NOT leak here? Are final fields automatically WeakReferences such that if the strong references to parent(s) are nulled, they are eligible for garbage collection?
The JLS does not seem to note anything special about final
Update:
So this question of mine was founded on the premise that "reachability" and strong/weak references are closely related. There is this confusing oracle doc on reachability that leads me to believe that nested references should always be "strongly reachable". Hence, I do null my nested object references in all my objects, but it appears that this obviously should not be the case from all of the comments I am receiving.
So regarding "reachability", then, is it simply just that nested object references are no longer considered "reachable" if parent references are no longer reachable?
It could be is true that the premise of this problem is incorrect, but there is still intriguing information to consolidate here.
As Makoto suggested, there is simply nothing special about final in variable declarations as far as GC is concerned. In your example code
private final Object obj = new Object();
will be garbage collected at the same time as
private Object obj = new Object();
Both are strong references, but are invalidated and garbage collected together with their parent class TestFinalGC instance. That is because when the instance is GC'd, the reference fields are destroyed as well and the references do not exist any more. obj's reference count thus decreases by one.
However, should you write something like
Object x = myTestFinalGC.obj; // only works if your obj is not private, of course
Then the object will not be garbage collected because it will still have one reference lingering around (assuming this particular line of code is in another class instance that remains alive when myTestFinalGC is garbage collected.
tl;dr: memory allocations are garbage collected when their hard reference count drops to zero (and the collector runs, of course). final doesn't change this fact.
I know an object reference or object handle has a size itself and the size is JVM dependent. I am just wondering what will happen after it has been explicitly set to null. Say we have a variable Object o = new Object() and later we set o = null. Will it be garbage collected?
In your example:
Object o = new Object()
o = null;
Assume o is either a field or a local variable/parameter. In the former situation it occupies 4/8 bytes in some other (outer) object. These bytes will be garbage collected when outer object is garbage collected.
If o is a parameter/local variable, no garbage collection is needed, the JVM will "recycle" the stack and simply reuse/erase that space.
References aren't objects and they are not subject to GC, other than in objects that contain them. They live in lexical scopes and they have the same lifetime.
No object will be garbage-collected while any kind of reference to it exists. If all the references that point to an object are set to null or are made to point to other objects, then the object may be garbage-collected, but it will no longer be "the object to which those references refer".
Note that Java has a class called WeakReference, which is designed to hold a reference to an object without preventing it from being garbage-collected. If when the system performs a garbage-collection there exists any object which is the target of a WeakReference, but would otherwise be eligible for garbage-collection, the system will invalidate the WeakReference. At that point, if nobody asked the WeakReference for its target before it was invalidated, there will no longer be any reference to the object, and it will thus be eligible for collection.